The dual role of a bHLH protein: Is SCRM1 / ICE1 an integrator of cold-stress and stomatal fate signaling?

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The formation of functional cell patterns in multicellular organisms requires specific cell-fate determinants. The outcome of this formation is influenced by the environment. Stomatal development in the plant Arabidopsis thaliana provides an easily accessible model to study the integration of environmental cues into developmental processes. Stomata are valves in the plant epidermis that regulate the gas exchange between the air and the intercellular spaces. The number of stomata per leaf area is influenced by CO2 and light intensity, and systemic signaling is thought to be involved in controlling stomatal development in response to these cues. Two basic helix-loop-helix (bHLH) transcription factors (TFs), SCRM1 and 2, play a key role in regulating the entry of protodermal cells into the stomatal lineage and in the progression through the stomatal lineage that ultimately forms mature stomata. SCRM1 interacts with three known bHLH TFs (SPCH, MUTE and FAMA) that positively regulate sequential steps during stomatal cell-state progression, but regulators of SCRM-activity in the stomatal lineage are unknown. The epidermis of dominant scrm-D mutants develops only stomata, so scrm-D provides a basis for a genetic screen for SCRM regulators. The finding that SCRM1 is ICE1, a regulator of freezing tolerance, suggests a dual role of SCRM1, which may act as an integrator of abiotic stress signaling and stomatal development. This project aims at unraveling the molecular mechanisms that regulate both functions of SCRM1 and at understanding the impact of cold signaling on stomatal development. It will give insight into how environmental cues are integrated into developmental processes. Three specific aims are: 1: Determine the role of systemic cold signaling in stomatal development. 2: Unravel mechanisms that regulate SCRM1 or ICE1 action on the post-translational level. 3: Identify novel factors that regulate the dual function of SCRM1.